This invention relates to apparatus for determining the location of an edge of a document, and in particular, but not exclusively, to a position sensing means for use in a banknote validator.
Various types of banknote validator are known. One such type is a banknote validator using an optical sensing head which illuminates a banknote and senses the spectral response of the banknote to determine whether the banknote paper and/or the printing inks are genuine. Such validators are described in U.S. Pat. Nos. 5,140,166 and 5,381,021. A banknote passing through a transport mechanism is sensed using an optical head. Because the transport mechanism pre-aligns the banknote with a lateral edge before the banknote is sensed, the lateral position of the banknote is known. The position of the banknote as it moves through the transport mechanism can be determined simply by sensing the leading and trailing edges of the banknote.
However, banknote validators using such a pre-alignment mechanism are able to process banknotes only at a limited rate due to the need for mechanical pre-alignment of banknotes entering the transport mechanism.
British patent application GB 2029007 describes a banknote validator incorporating a width and length detection arrangement. The banknote is passed through a light curtain consisting of a number of photosensors arranged in a line across a transport path for banknotes. The leading and trailing edges of the banknotes are sensed, the number of intervening scan pulses being counted in order to determine the banknote length. Meanwhile, the width of the banknote is determined by counting the number of darkened photodiodes after the banknote enters the light curtain. A serial diode signal (SDS), which is read out from a shift register, exhibits logic 1 and logic 0 states, and the number of 1s counted is a measure of the width of the banknote. The banknote is always prealigned, as the lower edge of the banknote moves on a sliding surface during movement through the light curtain.
In an arrangement described in U.S. Pat. No. 4,855,607, an array of photodiodes is used in conjunction with a deskewing mechanism for sensing the skew of a moving mail piece and then deskewing the mail piece. In an example given of a transmissive photosensor, the skew of the moving mail piece is detected by changes in the number and location of the individual photosensors which are occluded during each sampling period as the mail piece moves into the apparatus. The speed of asymmetric drive motors are varied according to the sensed skew, in order to align the mail piece with a transfer writing head.
In a further arrangement described in Texas Instruments product brochure xe2x80x9cShowcasexe2x80x9d issue 20, at page 9, a line imager is described which is suitable for the sensing of banknotes. The line imager is a charge mode imager, with the analog voltage per pixel proportional to the product of the light intensity on the pixel and the exposure time. The analog output voltages are shifted out serially, and 16 grey scales of output are possible. It is stated to be suitable for reading black and white documents, and measuring marks or edges.
According to one aspect of the present invention there is provided apparatus for determining the location of an edge of a document on a transport path, said apparatus comprising:
means for sensing points distributed across said transport path to produce a data series identifying said location by a transition occurring therein; and
processing means for dividing said series of data into a plurality of sets of data each comprising three or more data items and corresponding to a series of the sensed points, and for analysing said sets to detect said transition.
This aspect of the invention provides for increased processing speeds for the data produced by the sensing means.
It is therefore possible to provide a position sensing module for a high speed banknote validator which is able to supply the evaluation circuitry of such an mechanism with an accurate description of the edge positioning of a banknote as it enters a transport mechanism.
It is therefore also possible to provide a sensing means which is able to produce outputs from a shift register means receiving charges from an array of photodetectors and process those outputs at high speeds. The sensing module may therefore repeatedly scan a large number of pixels across the banknote in a relatively short period.
Preferably, the sensing means is for producing a plurality of contemporaneous outputs, said produced data is in the form of a plurality of data streams and said operating means is for operating on said data streams in parallel to detect a transition occurring in any of said data streams.
Such parallel processing of contemporaneous outputs from the sensing provides for a higher speed of processing than a conventional serial processing method.
According to a further aspect of the invention there is provided apparatus for determining the location of an edge of a document on a transport path, said apparatus comprising means for sensing points distributed across said transport path to produce a data series identifying said location by a transition occurring therein, and processing means for detecting said transition and selectively retaining a part of said data series at which said transition occurs, in response to detecting said transition.
This aspect of the invention provides for high speed preprocessing of the data produced by the sensing means, reducing the amount of data to be processed at a later stage to identify the edge locations of a document on the transport path.
Another aspect of the invention provides apparatus for detecting the location of an edge of a document moving along a transport path, comprising light source means, means for diffusing light from said light source means for back-lighting said document, a linear light-detector array for sensing light transmitted via said diffusing means, and focusing means for generally focusing said light-detector array both on said document and on the adjacent surface of said diffusing means.
This aspect provides for a high definition of transitions occurring at the document edges. The diffusing means can be used both to homogenize the light produced by said light source means and to provide a fixed focusing plane for the focusing of the light detector array.